Steering Gearbox
Motor and Power Assist Mechanism
A motor is housed inside the gearbox housing. It consists of a yoke with a permanent magnet fixed in it, a rotor with a
field coil, and brushes which pass electricity to rotor commutators. The motor is on the rack shaft of the steering gearbox.
The drive current from the control unit flows through the power relay and fail-safe relay to the brushes of the motor.
The drive current flows to rotor commutators through brushes, exciting the field coil of the rotor and rotates the rotor with
the magnetic action of the permanent magnet inside the yoke. The rotor rotates in the opposite direction when the direc-
tion of drive current is reversed.
The rotation torque of the motor is transmitted to the ball screw throught the helical gear. The rotation torque on the ball
screw is converted into assist thrust in the direction of steering rack by the recirculating ball screw. The mechanical advan-
tage of the recirculating ball screw, compaired to a conventional rack and pinion, acts to reduce the steering force to the
steering rack through the joint, to lessen the steering effort required at the steering wheel.
Recirculating Ball Screw
The recirculating ball screw is constructed so that steel balls roll between the screw shaft and the nut, and those coming
out of the screw face enter the screw groove again through a tube for recirculation. The rotation torque on the shaft is
converted into thrust in an axial direction as the slope of the screw shaft presses against the nut through the steel balls.
Since the torque conversion is made in this method through the screw shaft and the steel balls, the friction is small, high
transmission efficiency is obtained for both the forward and backward directions, and the steering feels identical to the
ordinary manual steering.
Furthermore, since the force from the backward direction (force pressing the steering rack) is converted into the rotation
torque of the screw shaft, instantaneous kick-back from the road surface will rapidly rotate the motor. In such a rapid rota-
tion, the inertia of the motor increases, which acts to reduce the kick-back. Any great kick-back will be transmitted from the
steering rack to the torque sensor on the pinion, which acts to twist the pinion shaft in the same direction as the input
direction of kick-back. As a result, the motor operates the kick-back to reduce assist force.ProCarManuals.com
